This invention relates to a method for terminating a reaction in a gas phase polymerization system.
It is of course well known that a significant portion of alpha-olefin homopolymer or copolymer, e.g., ethylene copolymer polymerizations are presently being conducted in fluidized bed reactors. Karol, et al. U.S. Pat. No. 4,302,566 discloses a typical fluidized bed reactor which is presently used for the preparation of ethylene copolymers in a fluidized bed reactor. As suggested in this patent, it is essential to operate the fluid bed reactor at a temperature below the sintering temperature of the polymer particles (Note column 12, lines 39-53 of Karol et al. U.S. Pat. No. 4,302,566).
In normal operation, the temperature of the fluidized bed is primarily controlled by passing recycle gas through a circulator e.g., a compressor and then through a heat exchanger, wherein the recycle gas is cooled to remove the heat of reaction before it is returned to the fluidized bed (Note column 11, lines 35-53).
If the circulator in the fluidized bed arrangement fails, e.g., due to electrical or mechanical failure, the cooling means for controlling the temperature in the bed becomes inoperative. Since olefin reactants are still in contact with active catalyst, exothermic heat of reaction causes the temperature of the bed to climb toward sintering temperatures in a run-away fashion.
Unfortunately, however, allowing the polymerization reaction to continue during emergency situations could cause severe future operating problems. In a gas phase fluidized bed polymerization reaction system, loss of fluidizing gas flow with continued reaction normally results in melting of the polymer powder and the formation of a molten chunk. The worst case would be caused by a complete loss of utilities--electric power, cooling water, instrument air, steam, etc.
As a result, when necessary, the art has resorted to various techniques for terminating a fluidized bed polymerization in the shortest possible time. Thus in one such technique, Stevens et al. U.S. Pat. No. 4,326,048 describes a method for terminating a gas phase olefin polymerization by injecting a carbon oxide. The injection of the carbon oxide may take place downstream of the polymerization reactor, e.g., in the recycle gas line (Note Column 4 lines 29-33 of the Stevens et al. U.S. Pat. No. 4,326,048).
In another technique, Charsley U.S. Pat. No. 4,306,044 discloses a means for introducing carbon dioxide into a gas-phase olefin polymerization system to at least reduce the rate of polymerization. For example, the carbon dioxide can be injected manually when the polymerization does not respond to other means of control. One other means of control is by rapid venting of the reactor as fast as possible in an attempt to control the run-away reaction. In this regard it is noted that the fluidized bed system as described in Karol et al. U.S. Pat. No. 4,302,566 is expressly provided with a venting system for shutdown. Venting systems of the prior art are relatively large and create additional problems.
Accordingly, these remedies are not entirely satisfactory because of economic and environmental considerations. Hence, there is still a need in the art for killing a reaction by techniques which do not require a large venting system.